The present invention relates to pulverulent or granulated blends that contain formamidinesulfinic acid (FAS), at least one anti-coating agent and/or finely divided silicic acid.
Formamidinesulfinic acid (FAS) is employed in the form of an aqueous alkaline solution for the purpose of reductive bleaching in the paper and textile industries. Whereas FAS is relatively sparingly soluble in water (about 27 g/l), under alkaline conditions up to 100 g/l FAS goes into solution as sulfinate. For the bleaching liquor the optimal quantitative ratio of FAS to lye (NaOH, Na.sub.2 CO.sub.3) amounts to around 2:1. It is a disadvantage that the salts of formamidinesulfinic acid rapidly decompose in aqueous solution. Alkaline bleaching solutions are therefore produced in the paper industry only a short time before being added in metered amounts, and they then have to be consumed as quickly as possible.
For the continuous production and metering of bleaching solutions consisting of FAS and NaOH a special metering system has been conceived, which is in service with the majority of FAS customers. These systems run for 24 hours a day (as needed, during the working season or non-stop for several months). Manpower for constantly monitoring this metering of FAS is not available, or is only available to a very limited extent. Whereas addition of the alkaline FAS bleaching solution functions well in the case of the metering system that operates continuously, the metering of FAS causes operational problems again and again: FAS cakes together in the feed hopper (shaft) and does not slide, resulting in diminished addition and also in compaction of FAS in the region of the screw. At the present time the incorporation of vibrators or pneumatic beaters in the shaft is intended to provide a remedy. By virtue of the fluctuating flowability/grain-quality, fluctuating metered amounts often occur at constant screw speed.
The storage of FAS also causes difficulties quite often, since FAS possesses only low thermal stability. For instance, the SADT (self-accelerating decomposition temperature) ranges at around 50.degree. C. Since, depending on storage conditions, temperatures clearly above 30 degrees have to be reckoned with again and again, problems with stability in storage occur in such cases. After just a few months a distinct decline in flowability occurs, proceeding as far as agglomeration (FAS is weakly hygroscopic), a yellow coloration and also an increase in the thiourea content. The latter is of particular significance if the limit of 0.1% thiourea (TU), which in many countries is subject to mandatory labelling, is exceeded.
Although many stabilizing agents that are current in the industry improve the thermal stability of FAS, they cannot be admixed, since they give rise to an unpleasant odour similar to that of hydrogen sulfide. Besides the poor flowability, in particular lime deposits in dissolving vessels and pipelines pertaining to FAS metering lines represent a problem. Due to the high pH value of the FAS bleaching solutions, current scale-inhibitors generally display a limited effect.
With a view to eliminating the lime deposits, the systems have to be switched off, flushed with mineral acids and cleaned.
An object of the present invention is to make available an FAS that remains flowable also in the event of storage and that prevents the formation of lime deposits in the course of its use. At the same time, it is an objective to increase the stability in storage as a result of stabilization of the FAS.